This invention relates to a method and apparatus for a multiline laser optoacoustic detection, and more particularly to an improved optoacoustic detector, and a method of using such an improved optoacoustic detector, for determining the combination of constituent gases in a sample, and determining the concentration in parts per million of each constituent.
Optoacoustic detectors have become very promising for gas analysis in recent years. See "Laser Optoacoustic Spectroscopy-A New Technique of Gas Analysis by L. B. Kreuzer," Analytical Chemistry, Vol. 46, No. 2., February 1974 pp 2394A-244A, and "Excited-State Spectroscopy of Molecules Using Opto-acoustic Detection" by C.K.N. Patel, et al., Physical Review Letters, Vol. 38, No. 21, May 1977, pp 1204-1207. The effort is now being made to improve the sensitivity of optoacoustic detectors.
Briefly, an optoacoustic detector operates by sensing pressure pulses induced in a gas sample by absorption of chopped laser radiation passing through it. The laser is tuned to different wavelengths to detect the presence of different constituent gases in the sample. Assuming that no energy is absorbed by the sample cell, the energy absorbed by the sample during each light pulse will increase the pressure of the gas in the sample cell in proportion to the amount of the absorbing gas present. This pressure pulse may be detected by a microphone, hence the term optoacoustic detector for the sample cell and microphone. The problem is that in practice the windows through which the laser beam passes through the cell will absorb some energy and introduce acoustical noise in the system. Efforts made in the past to cope with this acoustical noise have not been totally satisfactory. It would be preferable to devise an optoacoustic detector which eliminates the acoustical noise generated by heating of the windows.